(1) Field of the Invention
The present invention relates to an optical communication system, and more particularly, to an optical communication system using wavelength division multiple access (WDMA) and code division multiple access (CDMA) in an optical fiber section.
(2) Description of the Related Art
Services on a communication network have become increasingly diversified, and new services taking advantage of the network have been expanding. A representative example thereof is the integration of broadcasting and communication services such as so-called triple-play service for integrating broadcasting, internet and telephone (audio communication) services. This is a representative application of existing information services. The triple play is a keyword indicating a next generation network accommodating diversified information communication services.
Under this circumstance, FTTH construction with a passive optical network (PON) is in the mainstream in an access network. A PON system is comprised of an office side line terminal apparatus OLT (Optical Line Terminal) disposed in a station building of a communication carrier and a plurality of subscriber connection apparatuses ONU (Optical Network Unit) each disposed in a user's home. In the PON system, a single optical fiber (trunk optical fiber) is laid from the OLT to a service area, the trunk optical fiber is branched by a splitter into a plurality of branch optical fibers and ONUs are connected to the respective branch optical fibers, thereby to distribute signals to each user's home in a point-to-multipoint manner.
Since the PON system has a signal multicast function by optical branching, it is effective as an infrastructure for distributing a large amount of data such as a high-resolution image. Further, since a plurality of ONUs can share a trunk optical fiber, the PON system is advantageous in reducing the cost of laying optical fibers and the number of transmitting and receiving devices on the OLT side, compared to a star network for connecting a station building with user's homes in a point-to-point manner.
The current PON system includes ITU-T G(Gigabit-capable)-PON and IEEE GE(Gigabit-Ethernet)-PON. Details of G-PON are defined, for example, in ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (GPON): General characteristics”, ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification”, and ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification”) and details of GE(Gigabit-Ethernet)-PON are defined in IEEE 802.3ah “CSMA/CD Access Method and Physical Layer Specifications Amendment: Media Access Control Parameters, Physical Layers, and Management Parameters for Subscriber Access Networks.”
While attention is being given to the development of the integrated services of broadcasting and communication, enhancement of communication density (higher multiplexing), enhancement of communication speed (higher bit rate), and expansion of fiber laid areas are required for the PON system in order to distribute high-resolution images, for example, on high-definition television, to a larger number of users. PON-related standardizing organizations (ITU-T and IEEE) have started to study next-generation PONs subsequent to the current PON system.
Currently, in such standardization conferences, 10GE-PON and WDM-PON are proposed as next-generation PONs. As the multiplexing of the next-generation PON, time division multiple access (TDMA) is in the mainstream, as in the case of the current PON. Further, the application of code division multiple access (CDMA) is under study as another multiplexing method. Since CDMA can transmit and receive a plurality of flows with a same carrier wave at a same time, and it is not necessary to adjust communication timing among ONUs and provide a guard time between frames, CDMA has an advantage that information transmission efficiency with respect to transmission bandwidth is higher than that of TDMA. Further, in CDMA, transmission data is protected by spread spectrum with orthogonal spreading codes, therefore, in the PON system in which multiple users are accommodated on a same optical fiber, the effect of enhancing the confidentiality of information can also be expected.